(a) Field of the Invention:
The present invention relates to an apparatus for producing semiconductor devices by the liquid phase epitaxial technique and more particularly to a structure for dissipating heat from a solution of semiconductive material in molten metal solvent through a substrate.
(b) Description of the Prior Art:
Thre have been developed two types of solution growth techniques for producing semiconductor devices, one of which is called "a cooling method" wherein a surface of the substrate is brought into contact with a solution of a semiconductive material in a molten metal solvent and the temperature of the solution is lowered during the deposition of the semiconductive material on the substrate. The other is a very recently developed technique called "a temperature difference method" in which, as disclosed in my copending application Ser. No. 862,978 filed Dec. 21, 1977 and its abandoned parent and grandparent applications Ser. Nos. 561,092 filed Mar. 24, 1975 and 314,448 filed Dec. 12, 1972 (the contents of all three of which are incorporated hereinto by reference), a constant temperature difference is established between an upper portion and a lower portion of a well or reservoir containing the solution, and the deposition of epitaxial layers on the substrate is conducted without requiring any lowering of the temperature of the solution.
In the cooling method, if the solution cools faster than the surface of the substrate, the deposited epitaxial layer becomes non-planar. In order to overcome this difficulty, U.S. Pat. No. 3,665,888 issued to Bergh et al teaches the use of heat sink means, through which flows a cooled inert gas, under a substrate-accommodating portion of a slider to establish a vertical temperature gradient via the heat sink. Similarly, U.S. Pat. No. 3,785,884 issued to Lockwood et al discloses a container provided with a heat radiation window therethrough with a substrate seated over the window and a plate of sapphire or quartz placed between the substrate and the window. However these approaches to overcome the difficulty are not so effective to obtain a stable and planar growth of the epitaxial layer when they are applied to the apparatus for successive growth of multilayer structures on a plurality of substrates by using the temperature difference method, because in the latter method the deposition end of the well is maintained at a predetermined temperature lower than that of the upper end thereof and a constant temperature difference is maintained between both ends during the deposition of epitaxial layers so that a much higher amount of heat dissipates through a region on the periphery of the substrate compared with that in the cooling method.
For example, the cooling heat sink proposed by Bergh extends very far from the substrate-containing region of the slide so that it absorbs a much higher amount of heat through a region on the periphery of the substrate than that through the substrate itself, when it is used in an apparatus according to the temperature difference method. The window and a sapphire or quartz plate proposed by Lockwood are rather less heat-conductive than carbon. Therefore, they adversely affect the heat dissipation through the substrate. On the other hand, in the temperature difference method, Akai teaches the use of heat sink as disclosed in U.S. Pat. No. 3,933,538. However, this heat sink is provided on the bottom surface of the sliders over a very wide range such as a distance including three substances, so that it can not have any effect of dissipating most of the heat through the substrate.
Moreover, Bergh teaches the art that, after a thermal equilibrium has been established between a substrate and a source-saturated melt, a cooled inert gas is caused to flow through a heat exchanger to produce a reduction of the temperature of the substrate. Hence, for each replacement of a crystal-deposited substrate by a fresh substrate of which a crystal is to be epitaxially deposited, Bergh must repeat his cycle which consists of: heating to achieve the thermal equilibrium and cooling to cause a reduction in temperature. Such repetition of the heating-cooling cycle in Bergh is very troublesome in practice.
U.S. Pat. No. 3,762,943 issued to Winstel et al teaches the use of a finger type of heat sink in a vertically extending apparatus which creates a thermal resistance which increases toward the center of this finger. The temperature gradient is established only in the axial directions and not in the radial direction. This technique cannot be applied to an apparatus of the horizontally-extending type used for effecting the temperature difference method to which the present invention is directed, because the condition of heat flow through the apparatus is entirely different from that in the Winstel apparatus. Thus, in the horizontally-extending type apparatus where a temperature difference is maintained between the upper portion and the lower portion of a melt according to the temperature difference method, the thermal resistance at the center of the substrate between the substrate and the heat sink must be as small as possible as compared with the thermal resistance on the outside of the periphery portions of the substrate so as to effect a vertical flow of heat substantially only through the substrate. This is entirely the reverse of the Winstel teaching.